The bottom rail closes off the bottom of a gate frame. On a swing gate it usually matches the rest of the frame, for example a 50x25 tube all the way round, and just gives the lower battens or slats something to fix into.
On a sliding gate the bottom rail is doing far more work. It carries the bottom rollers in tracked builds, or it sits over the cantilever rail (which is often welded inline as the bottom rail itself) in cantilever builds. So sliding-gate bottom rails are usually heavier: 100x50 RHS or larger, where the swing-gate version of the same gate would use 50x25.
CAD60 surfaces top-rail and bottom-rail sizes as separate inputs on sliding-gate models so the bottom rail can be specified larger than the rest of the frame without affecting the rest of the geometry.